You might want to change the value of an object's air resistance if it changes its size. Let's say we're making an object using the hoverdoc sprites. We'll have it start out curled up, then unfold on click. Air resistance is proportional to surface area - in the hoverdoc's case this will be different depending on which way it's moving, but let's just average the height and width and call that the surface. What follows is a complete working example agent.

CEE's use of AERO is close to real-world air resistance. In rough models, deceleration due to air resistance is usually taken to be proportional either to speed, for low speeds, or to the square of speed, for higher ones.